As a veterinary pathologist, I can assure you that we are not always able to determine a cause of death. Sometimes the best we can do is 'rule out' the common things that you or your veterinarian could actually do something about. It's as frustrating for me as it is for the producer and their veterinarian.
I can't even begin to hazard a guess as to why your bull is dead, 243, unless you've got a lot of clover in your pasture and he's a 'frothy bloat' case... but I can't tell that just from a photo of the animal.
The brainstem sample had nothing to do with determining cause of death. It's part of USDA/APHIS' ongoing 'passive surveillance' for Bovine Spongiform Encephalopathy, aka Mad Cow Disease.
We routinely collect brainstem on every cow/bull over 20 months of age that comes through the diagnostic lab here, and submit for testing for BSE. There are federal funds available to support this surveillance... I don't see any of the $$$ they pay us, and don't know what they're currently reimbursing veterinarians and producers for collection and submission of samples, but the $100 payment is intended to help cover expenses associated with proper disposal of the carcass (not dropping it off in a sinkhole or dragging it off to the far end of the farm and leaving it for scavengers).
Blackleg would not be on my radar screen for a mature animal - provided they had a history of adequate calfhood vaccination against the Clostridium group of pathogens. The spores are in the soil EVERYWHERE, and can remain viable virtually forever! (40 million years+) Disturbance from the excavator probably doesn't increase risk significantly; they're already there.